Ultrasound consists of propagating mechanical oscillation with a frequency (>20 kHz) exceeding hearing range. Longitudinal waves are generally used in medical ultrasound applications. The longitudinal wave can propagate through all types of media, gases, fluids and solids, which makes it useful in various applications. As the ultrasound travels through a medium, it becomes attenuated due to absorption, reflection and scattering. Thus, material properties of the medium alters the characteristics of the ultrasound propagation and recorded ultrasound signal can be used to evaluate material properties.
In many applications, ultrasound has to travel through unknown composition of acoustically known materials before reaching the region of interest (the object under evaluation). In these cases, the effect of overlaying materials has to be considered and eliminated prior to successful ultrasound evaluation of the object of interest. As an example, diagnostic evaluation of bone using ultrasound necessitates conduction of the ultrasound measurement through the soft tissues overlying bones.
Many industrial processes depend on the measurement of composition and thicknesses of two or more materials. Commercially available ultrasound thickness meters measure thicknesses of two or more materials but these techniques need acoustically visible interfaces (i.e. ultrasound reflections) between different material layers and the prior knowledge of the order of materials in the layered structure. A close monitoring of material characteristics can increase efficiency and improve the product quality. Additional benefits are likely to occur, if such measurements can be accomplished rapidly and in a non-destructive way with acceptable accuracy and precision. Material properties are traditionally measured by some form of destructive analysis (sample separation, peeking) but may be analysed acoustically by using ultrasound measurement and signal analysis.